1. Field of the Invention
The present invention relates to an image processing method and an image processing apparatus used for a thermal transfer recording mode of performing thermal transfer recording by using, e.g., a linear thermal head having a plurality of heat generators linearly arranged therein. The present invention also relates to a printed matter crated by using the image processing method and the image processing apparatus.
2. Description of the Related Art
As a method of recording a facial image in an image display unit having a facial image for personal authentication therein, e.g., various kinds of certificates, credit cards, or membership cards, a sublimation type thermal transfer recording method conventionally forms a mainstream. According to this sublimation type thermal transfer recording method, a thermal transfer ribbon obtained by coating a film-like support with a dye having sublimation properties (or heat transient properties) is superimposed on a recording target medium having an accepting layer that accepts the sublimation dye, the thermal transfer ribbon is selectively heated by, e.g., a thermal head based on original image data to be recorded, and a desired image is thereby subjected to sublimation transfer recording on the recording target medium.
In this sublimation type thermal transfer recording method, it is generally widely known that a color image that is rich in gradation properties can be easily recorded. However, in the sublimation type thermal transfer recording method, there is a drawback that materials that can be colored with a sublimation type material is limited and this method can adapt to limited recording target mediums only. Further, in general, the sublimation type dye is poor in image durability, e.g., light-resisting properties or solvent-resisting properties.
On the other hand, according to a fusion type thermal transfer recording method, a thermal transfer ribbon obtained by coating a film-like support with a material having a color pigment or a dye dispersed in a binder, e.g., a resin or a wax is selectively heated, and this ribbon is transferred together with the binder onto a recording target medium, thereby recording a desired image.
In this fusion type thermal transfer recording method, an inorganic or an organic pigment that is said to generally have excellent light-resisting properties can be selected as a color pigment. Further, in the fusion type thermal transfer recording method, an ingenuity can be exercised with respect to a resin or a wax used in a binder. Therefore, in the fusion type thermal transfer recording method, solvent-resisting properties can be improved. Furthermore, in the fusion type thermal transfer recording method, any recording target medium having adhesion properties with respect to a binder can be basically used. This method has an advantage, e.g., extensive selection of recording target mediums as compared with the sublimation type thermal transfer recording method.
However, the fusion type thermal transfer recording method uses a dot area gradation method of varying a size of transferred dots to perform gradation recording. Therefore, in order to accurately control a dot size to perform multi-gradation recording, various ingenuities are required. For example, there is a method of aligning arrays of pixels (dots) to be transferred in a staggered pattern to perform recording (which will be referred to as an alternate driving method hereinafter). When this alternate driving method is used, thermal interference of adjacent heat generators in a thermal head can be reduced, and a dot size can be controlled without being affected by adjacent pixels, thereby performing excellent multi-gradation recording.
Further, on a recording medium, e.g., an ID card is recorded a fluorescent image formed by using a transparent and colorless ink including a fluorescent pigment excited by ultraviolet light or the like in some cases. Furthermore, such a fluorescent image may be printed as continuous images (all pixels are printed) around a region printed by the alternate driving method. Such printing is intended to have an effect of causing a periphery of a fluorescent image (a region where continuous images are printed) to intensively emit light for provision of contrast, thereby improving an appearance. Such a technique is generally widely known.
However, the above-explained conventional technology has the following problems.
As explained above, in the alternate driving method, respective pixels (dots) constituting an image are rearranged into a staggered pattern to form an image. Therefore, pixel information of a part to which dots are not transferred is lost. In a multi-gradation image like a facial image, even if pixel information is lost in a staggered pattern, information as a facial image is not lost. However, in a binary image, e.g., a character or a geometric pattern, when dots are transferred in a staggered pattern, pixel information of a part to which dots are not transferred is lost, and there is a possibility that the image does not function as a character or a geometric pattern.
Moreover, in a printed matter, e.g., an ID card, various images are superimposed and printed to improve appearance in some cases. For example, a different image may be superimposed and printed on a background image, e.g., a fluorescent image. Additionally, a different image may be superimposed and printed on a fluorescent image including a region where all pixels are printed and a region printed by the alternate driving method. In such a case, in the fusion type thermal transfer recording mode, a printing state varies depending on each region where various images are superimposed. That is, when an image state or a recording medium state partially varies, printing the image with a uniform energy results in a problem that a region where a desired image cannot be obtained is present in the image (a printing result) printed on the recording medium.